4 Results on individual systems

4.1 J2233-606, z $_{\mathsfsl{abs}} \simeq {\mathsfsl{1.92}}$

The system at $z_{\rm abs} \simeq 1.92$ toward the quasar J2233-606 has saturated hydrogen lines (from Ly-$\alpha$ up to Ly-8) and metal lines of C II, Si II, Si III, C IV, and Si IV. The results obtained with the MCI are presented in Table 1 and illustrated in Figs. 1 and 2. Parts of profiles included in the $\chi^2$ minimization are marked by horizontal lines at the panel bottoms in Fig. 1. Profiles of the doublet N V $\lambda1238$ Å and N V $\lambda1242$ Å were calculated later using the obtained velocity and density distribution and the metallicity derived from the fitting of N III $\lambda989$ Å. It is seen from Fig. 1 that most spectral features can be well represented assuming uniform metallicities and a common HM UV background. Figure 2 demonstrates the distribution of the radial velocity and gas density (panels a and b) along the line of sight (rearranged in accord with the principle of minimal entropy production rate). The density distributions for the ions involved in the optimization are shown in panels c-j, whereas the density-weighted velocity distributions which determine the shapes of the spectral lines are presented in Fig. 3. This figure shows that the density-weighted velocity distributions for low ions C II and Si II are similar, but differ from those for high ions C IV and Si IV. These distributions easily explain why the lines of C II and Si II look very much alike and why their centers are displaced by $\Delta v \simeq 4$ km s^-1 (DP) with respect to C IV and Si IV.

The study of this system by DP, who used the standard Voigt profile fitting, produced comparable column densities (albeit 20-50% smaller). However, the metallicities obtained by DP for two main clouds (at v = 0 km s^-1 and v = -43 km s^-1) differ nearly by two orders of magnitude: [X/H] = -0.9 and -2.7, respectively (in our case [X/H] $\simeq -2.2$ for the whole system). As shown in Paper I, the Voigt fitting may in general yield correct column densities when applied to unsaturated lines, but the mean U and, hence, the ionization corrections may not be unambiguous. Therefore, the conclusion made by DP that the $z_{\rm abs} \simeq 1.92$ system contains "a region of intense star-formation activity'' may not be well justified since this result is model dependent.

Figure 5: Computed velocity (upper panel) and gas density (lower panel) distributions along the line of sight for the system at z = 1.942616toward J2233-606. Shown are patterns rearranged according to the principle of minimum entropy production rate (see text).

The values of the average gas density n₀ and kinetic temperature $T_{\rm kin}$, and the cloud thickness Lestimated in our model (see Table 1) are typical for the Ly-$\alpha$ systems discussed in the literature (e.g., Giallongo & Petitjean 1994; Viegas et al. 1999; Prochaska & Burles 1999; Chen et al. 1998; Chen et al. 2001). Low metallicity for the whole system ([X/H] < -2.0) and its dimension of 20 kpc imply that this system can originate in a galactic halo or in a large scale structure object.

4.2 J2233-606, z $_{\mathsfsl{abs}} \simeq {\mathsfsl{1.94}}$

This system exhibits a plenty of metal lines in different ionization stages. The metal profiles are not very complex and extend over the velocity range from -100 km s^-1 to 100 km s^-1. Results obtained with the MCI are presented in Table 1 and shown in Figs. 4 and 5. As in the previous system, most absorption features can be well described with uniform metallicities and a common HM spectrum. The Ly-$\alpha$ profile is contaminated by the forest absorption in the blue and red wings and therefore the Ly-$\alpha$ absorption feature was not involved in the analysis. The profiles of Mg II $\lambda2796$ Å and Al II $\lambda1670$ Å were computed later using the derived velocity and density distributions. Mg II $\lambda2796$ Å is contaminated by a telluric line and this explains the difference between the computed and observed profiles. The synthetic and observed profiles of Al II $\lambda1670$ Å show much more pronounced discrepancy. Fractional ionisation curves for Al II and Al III were computed with CLOUDY. These curves allowed us to fit the Al III doublet quite well with the Al abundance similar to that obtained for the other metals. However, when the Al II profile was included in the fitting, the Al metallicity differed by order of magnitude from the other metals. Besides it was impossible to fit adequately the Al III doublet. Similar behaviour of Al was reported also by DP who noted that "the recombination coefficients used to compute the aluminium ionisation equilibrium [in CLOUDY] are probably questionable''.

Column densities derived by DP coincide well (within 15%) with that obtained in our procedure except for the saturated Si III $\lambda1206$ Å line for which the Voigt fitting gave nearly 2 times lower value. The abundances estimated in DP scatter again from component to component, but nevertheless they conclude that "the gas in this system is likely of quite high metallicity (larger than 0.1 solar)''.

Figure 6: Same as Fig. 1 but for the $z_{\rm abs} \simeq 1.87$ system toward J2233-606. The zero radial velocity is fixed at z = 1.87008. The corresponding physical parameters are listed in Table 1. The normalized $\chi ^2_{\rm min} = 1.60$ (the number of degrees of freedom $\nu = 1158$).

Similar to the Voigt fitting, the MCI also delivered for this system high metal abundancies: one third solar for carbon and silicon and nearly two times lower for nitrogen, magnesium and aluminium. Taking into account this result and a compact dimension ($\simeq$5 kpc, see Table 1) of the absorbing region we come to the same conclusion as Prochaska & Burles (1999) did: the system at z = 1.94 can hardly be a large scale structure object (like a filament or a wall) and should be related to a galactic system (may be a region of intense star formation).

4.3 J2233-606, z $_{\mathsfsl{abs}} \simeq {\mathsfsl{1.87}}$

This is the most interesting system from the family of the absorbers at z = 1.9 toward J2233-606. The metal line profiles show a rather complex structure extending over the velocity range of about 700 km s^-1. Some of these profiles are severely blended that hampers the unique Voigt profile deconvolution (e.g. DP assumed 17 components to describe metal profiles).

The MCI code turned out to be much more robust and was able to recover the self-consistent line profiles even under such unfavourable conditions. The physical parameters which the MCI delivered for the z = 1.87 system together with the underlying velocity and density distributions are presented in Table 1 and in Figs. 6 and 7. It is seen from Fig. 6 that like in the previous two systems all lines are well described with a single parameter set, uniform metallicities and a common HM UV background. The blue wing of the Ly-$\alpha$ line is contaminated by the forest absorption as is clearly seen from the Ly-$\beta$ and Ly-$\gamma$ profiles. The synthetic profile of the O VI $\lambda1031$ Å line was calculated later using the derived best fitting parameters and the oxygen abundance [O/H] = -1.0(which is about 3 times over the other element abundances from this system). Even with the increased abundance the synthetic profile of O VI is still much weaker than the observed intensities. This discrepancy rules out the ionization of O VI by the adopted background radiation. Taking into account that all other elements have been well described with a given HM spectrum and that the collisional ionization of oxygen can hardly be effective at low densities ( $n_{\rm H} \sim 10^{-5}$) and temperatures of $\sim$$25\,000$ K, this result seems to favor the interpretation that the O VI ion and the other ions do not arise in the same gas (Kirkman & Tytler 1999; Reimers et al. 2001).

According to our results, the absorber at z = 1.87 could be a large size cloud with very high velocity dispersion. Its estimated linear size of 80 kpc is consistent with dimensions of extended gaseous envelopes observed around galaxies at z < 1. In these envelopes, Mg II absorption is the dominant observational signature at the distancies up to a few tens of kiloparsecs (Bergeron & Boissé 1991), whereas highly ionizied species like C IV are observed at distances of at least 100 kpc from galactic centers (Chen et al. 2001). Since the extended structure of the same order of magnitude is observed at z = 1.87, we may conclude that this system arises in the external halo at large galactocentric distances.

Figure 7: Same as Fig. 5 but for the $z_{\rm abs} \simeq 1.87$ system toward J2233-606.